Distributed strain sensing based on hybrid system combining dual-sideband pump BOTDA and frequency-scanned <i>ϕ</i>-OTDR

A distributed absolute strain sensing technology based on hybrid system combining dual-sideband pump BOTDA and frequency-scanned <i>ϕ</i>-OTDR is proposed, which can achieve high resolution in a large range of strain and maintain high accuracy. The synthetic absolute strain is consisted by the relative strain and the initial absolute strain. The Brillouin gain spectra and Brillouin loss spectra are simultaneously obtained by designing the dual-sideband pump pulse scheme in the hybrid system, which are combined to construct the composite spectra. And then, the demodulation accuracy of absolute strain is improved by 1.76 times through extracting Brillouin frequency shift from the composite spectra. The relative strain with high accuracy and resolution is measured by frequency-scanned <i>ϕ</i>-OTDR. In the hybrid system, the dual-sideband pump BOTDA part determines the range of absolute strain measurement, and the frequency-scanned <i>ϕ</i>-OTDR part provides high resolution of strain measurement. The design of hybrid system realizes the reuse of devices to the greatest extent. The results indicate that the hybrid system achieves the absolute strain measurement in the range of 30 <i>μ</i><i>ε</i> to 3820 <i>μ</i><i>ε</i> with the resolution of 2 <i>μ</i><i>ε</i> and the accuracy of 3.204 <i>μ</i><i>ε</i>(0.1458 MHz). The proposed method provides a high-performance absolute strain sensing method for structural health monitoring.